Sforno on Genesis 17:16/ספורנו על בראשית י״ז:ט״ז

1וברכתיה, both during her pregnancy, the act of giving birth, and in the raising of the son, all of these being painless. This is the exact reverse of the curse G'd decreed on Chavah and women in general, when He said בעצב תלדי בנים, "you will give birth to children in pain and sorrow." (Genesis 3,16) Sarah was promised to be spared that curse which other women are afflicted with. This may be the meaning of Isaiah 29,22 אשר פדה את אברהם, "who has redeemed Avraham i.e. from the painful experience associated with raising children. (Sanhedrin 19)